1. Field of the Invention
The present invention relates to circuit protection technology of a transformer and more particularly, to a transformer failure analysis system.
2. Description of the Related Art
A transformer is an important power transmission and distribution equipment in a power system. Therefore, it is necessary to protect the operation of the transformer in a power system and to maintain its stability. However, when a power system applies a voltage to the transformer thereof, inrush current, i.e., transient over-current and over-voltage may occur due to magnetic saturation of the transformer iron cores. The inrush current may be 5˜10 times or more over the rated current, and will be attenuated to normal excitation current after several cycles or more than one minute. It causes impact and an adverse effect to the service life of related circuit components, and may be erroneously judged by a conventional suppress circuit to be a fault current of a short circuit. In this case, the suppress circuit will trip off the whole circuit, resulting in waste of time and labor in security examination and delay of normal production. When compared internal fault current with inrush current, inrush current has a large amount of waveform distortion of high harmonic components and even harmonics.
According to current power system protection circuits, recognition between an inrush current and a fault current recognition is done subject to the rule that when the second harmonic wave is over a predetermined rate relative to the fundamental wave (normally 17%), the current is determined to be an inrush current and the suppression protection circuit trips off the circuit; when the fundamental wave is higher than the rated current, the current is determined to be a fault current and the suppression protection circuit trips off the circuit within one cycle to protect the circuit and to avoid a malfunction. Thus, the aforesaid prior art technique avoids the protection circuit from a malfunction upon an inrush current, however the related circuit components must receive the impact of the over-voltage of several cycles of harmonic resonance and inrush current, affecting the service life and safety level of the whole circuit.
Further, conventional harmonic wave analysis is based on a periodic signal to analyze the waveform after fast Fourier transformation (FFT). However, because a transformer inrush current is a transient high harmonic signal, the analysis result is neither accurate nor reasonable, causing a blind point in protection circuit design and transformer protection ineffectiveness. Further, following the market trend to increase transformer capacity and to use high flux silicon steel for iron core, it can be seen that the value of the inrush current becomes relatively greater, the waveform distortion rate becomes relatively smaller and the second harmonic ratio becomes relatively lower. On the other hand, increase of system static capacitance subject to enlargement of ultra-high voltage system and increase of phase modulation equipment causes increase of the distortion rate of the waveform of the fault current in the circuit, complicating discrimination between fault current and inrush current.